ADIABATIC SEMIEMPIRICAL PARAMETRIC METHOD FOR COMPUTING ELECTRONIC-VIBRATIONAL SPECTRA OF COMPLEX-MOLECULES .1. POLYENES AND DIPHENYLPOLYENES

A parametric semi-empirical method for the calculation of the vibratio nal structure of the electronic spectrum and the determination of the parameters of the molecular excited state potential surface has been d eveloped. The method is based on the adiabatic molecular model and is unique for all sets of parameters of the excited states (first and sec ond derivatives of the matrix of coulombic and resonant one-electron i ntegrals with respect to the internal coordinates). Simplified analyti cal expressions for the changes in the molecular potential surfaces on excitation, which account only for the first-order terms, are obtaine d. It is shown that the parameters possess distinct local properties a nd may be transferred in a homologous series of molecules. The number of most significant parameters, sufficient to describe the molecular m odel adequately and to obtain satisfactory quantitative results, is ve ry small. Calculations of geometry changes and vibronic spectra for so me polyene and diphenylpolyene molecules using only two parameters sho w good quantitative agreement with experimental data. It is possible t o create a special data bank of molecular fragments for vibronic spect roscopy with relatively small structural groups (e.g. H->C= for polyen es and related compounds) and to use it to compute the excited state p roperties of complex molecules and their vibronic spectra employing th e suggested parametric method. (C) 1997 Elsevier Science B.V.